In order that plasma-arc welding of steels in carbon-dioxide environment or in the atmosphere of carbon-dioxide-based mixtures could compete with other widely known methods of welding, such as consumable electrode submerged-arc welding or consumable electrode carbon-dioxide-shielded arc welding, or consumable electrode carbon-dioxide-based mixture shielded arc welding, the non-consumable electrodes for plasma-arc welding in the atmosphere of carbon-dioxide or carbon-dioxide-based mixtures are required to provide an operating life sufficient for commercial use, with electric arc currents in excess of 400 A.
For example, when welding low carbon and low alloy steels without edge preparation, in order to obtain speeds of plasma-arc welding in carbon-dioxide or carbon-dioxide-based mixture atmospheres equal to or exceeding the welding speeds when using known methods, the non-consumable electrodes for plasma-arc welding are required to provide an operating life suitable for commercial use, with electric arc currents ranging from 400 to 1000 A.
Known in the art is a non-consumable electrode for plasma arc treatment in an atmosphere of chemically reactive plasma-generating gases including carbon dioxide, comprising a liquid-cooled metal holder made of copper and an insert of zirconium, metallurgically bonded thereto (cf. U.S. Pat. No. 3,198,932).
As this non-consumable electrode operates in a carbon-dioxide atmosphere, the reactive insert of zirconium chemically reacts with active components of plasma-generating gas, i.e. carbon and oxygen. Over the entire surface of the insert a film is formed consisting of zirconium-carbon and zirconium-oxygen compounds, which will be referred to below as zirconium oxycarbide layer. The film of zirconium oxycarbide exhibits a high thermal stability and improved emission properties as compared to metallic zirconium. The above non-consumable electrode has found application in apparatus for plasma-arc cutting and welding of metals with electric arc currents of up to 300 A.
In plasma-arc-carbon-dioxide-shielded arc treatment with electric arc currents exceeding 300 A, the operating life of such an electrode, however, is not suitable for commercial use. This actually prevents its being employed for plasma-arc welding of low carbon and low alloy steels more than 6 mm thick in an atmosphere of carbon-dioxide or carbon-dioxide-based mixtures.
Also known in the prior art is a non-consumable electrode for plasma-arc treatment of metals in chemically reactive plasma-generating atmospheres containing oxygen, nitrogen and/or carbon.
This non-consumable electrode comprises a holder of copper or copper alloys and an insert of hafnium (cf. U.S. Pat. No. 3,592,649).
As this non-consumable electrode is operated in a carbon-dioxide atmosphere, the insert of hafnium chemically reacts with active components of the plasma-generating gas, i.e. carbon and oxygen. Over the entire operating surface of the active insert a layer of the hafnium oxycarbide is formed which exhibits a higher thermal stability and improved emission properties than the zirconium oxycarbide layer.
The operating life of such a non-consumable electrode enables it to be employed for plasma-arc treatment in a carbon-dioxide atmosphere with electric arc currents of up to 400 A. It is higher than the operating life of electrodes having an active insert of zirconium.
For higher electric current applications, e.g. for electric arc currents in excess of 400 A, however, also becomes impractical on account of an increased rate of electrode erosion.
Thus, the non-consumable electrode with an active insert of hafnium cannot be used for plasma-arc welding of low carbon and low alloy steels having a thickness exceeding 6 mm in an atmosphere of carbon-dioxide or carbon-dioxide-based mixtures.
Another prior art cathode for electric arc processes in chemically reactive atmospheres includes a copper holder and an active insert composed essentially of hafnium and alloying additions of various metals and/or their oxides.
This cathode exhibits a lower rate of erosion as compared with the electrode having an active insert of pure hafnium only when the electric arc is operated in an intermittent cycle mode. For all operational modes, however, the operational life of such a cathode enables it to be used in plasma-arc treatment in a carbon-dioxide atmosphere with electric arc currents not exceeding 400 A.
Thus, the cathode with an active insert of hafnium with alloying additions cannot be employed for plasma arc welding of low carbon and low alloy steels over 6 mm thick in an atmosphere of carbon dioxide or carbon-dioxide-based mixtures.
The abovementioned electrode disclosed in U.S. Pat. No. 3,198,932 is produced by placing a chemically pretreated active insert of zirconium into the copper holder, simultaneously heating holder-and-insert combination to provide a metallurgical bond between the active insert and the copper holder.
The remainder of electrodes described above are manufactured by mechanically placing the active insert into the copper holder using such methods as cold stamping or other similar methods.
In all the electrodes described above the film of oxycarbide of the corresponding material of an active insert is obtained as an electric arc burns in a carbon-dioxide atmosphere.